This invention relates to hydrofoil craft, and particularly to hydrofoil craft capable of operating at high speeds with low drag.
Hydrofoil craft are boats which typically possess a more or less conventional planing boat hull and which have one or more support arms extending from beneath the hull into the water. One or more foils for supporting the hull are connected to the lower ends of the support arms. When the hydrofoil craft has accelerated to a sufficient velocity through the water, the lift created by the foils raises the hull above the water's surface, thus eliminating the hull's resistance.
Conventional hydrofoil craft have a number of problems which make them difficult or impractical to operate at high speeds. A first problem is cavitation, which is a phenomenon in which vapor bubbles form along the upper surface of a foil due to a low fluid pressure on this surface. Cavitation invariably occurs in conventional hydrofoil craft above a certain operating speed (typically around 50 knots). When the vapor bubbles caused by cavitation collapse in the water, they produce strong shock waves. If the collapse occurs in the vicinity of the foil, the shock waves not only produce unpleasant noise and vibrations, but can also physically damage the foil of the craft by pitting.
In order to prevent damage by cavitation, foils referred to as supercavitating foils have been developed. With a supercavitating foil, a large vapor-filled cavity, referred to as a separation bubble, is formed over substantially the entire upper surface of the foil. Vapor bubbles in the cavity are carried beyond the trailing edge of the foil and collapse in the water aft of the foil, so that shock waves produced by the collapse of the bubbles have much less effect on the foil than in a normal cavitating foil.
While a supercavitating foil prevents the collapse of air bubbles in the vicinity of the foil which could damage the foil, in order to prevent the separation bubble from collapsing, it is necessary to maintain the foil at an extremely high angle of incidence. This high angle of incidence results in a great deal of drag, so that the lift/drag ratio of a conventional supercavitating foil is so low as to make such a foil impractical. For this reason, supercavitating foils are not used in practice, and hydrofoil craft must rely on conventional cavitating foils, which as described above are unsatisfactory.
Aside from the problem of cavitation, conventional hydrofoil craft have the problem that their foils invariably operate in a turbulent flow regime, so that the drag on the foils is high, and a great deal of power is required to drive a conventional hydrofoil craft at high speeds.
Another problem is that conventional hydrofoil craft have their foils connected to the hull of the craft by support arms rigidly attached to the hull. As a result, all of the accelerations imparted to the foils are also imparted to the hull. The higher the speed of the craft, the greater the vertical accelerations of the foil, and even at moderately high speeds, if the foil is rigidly connected to the hull, the accelerations imparted to the hull may far exceed those that a human passenger can comfortably withstand.